Generally, polycarbonate/styrene containing copolymer blends can have high notch impact strength and good processability. Therefore, polycarbonate/styrene containing copolymer blends have been widely used in large injection molded products, such as computer housings or other office equipment, which emit a lot of heat. Accordingly, flame retardancy, heat resistance and high mechanical strength are important factors that should be considered when manufacturing the blend.
Halogen-containing flame retardants and antimony compounds are conventionally used to impart flame retardancy to resin compositions. However, halogen-containing flame retardants can produce toxic gases during combustion. Therefore, there is an increasing demand for non-halogen-containing flame retardants.
Currently the most common method for imparting flame retardancy without halogen-containing flame retardants uses a phosphorus compound. A representative phosphorus compound used as a flame retardant is a phosphate ester flame retardant, which is commercially available for purchase. U.S. Pat. No. 4,692,488 discloses a thermoplastic resin composition containing a non-halogen aromatic polycarbonate resin, a non-halogen styrene-acrylonitrile copolymer, a non-halogen phosphorous compound, a tetrafluoroethylene polymer, and a small amount of ABS copolymer. U.S. Pat. No. 5,061,745 discloses a flameproof resin composition containing an aromatic polycarbonate resin, an ABS graft copolymer, a copolymer and a monomeric phosphate ester. However, the resin compositions described above can require the addition of an excessive amount of phosphate ester flame retardant in order to obtain a certain level of flame retardancy. Furthermore, the flame retardant can migrate to a molding surface during a molding operation, which can result in surface cracking, i.e. the so-called “juicing” phenomenon, and further heat resistance of the resin composition can be deteriorated.
Other phosphorus compound flame retardants include phosphazene compounds. EP Patent No. 728,811 discloses a flameproof resin composition containing an aromatic polycarbonate resin, a graft copolymer, a vinyl copolymer and a phosphazene compound. EP '811 shows that no dripping occurs during combustion when using the phosphazene compound as a flame retardant even though an additional anti-dripping agent is not employed, and that the flameproof resin composition disclosed therein can have excellent heat resistance and impact strength. However, in EP '811, when using the phosphazene compound as a flame retardant, an increased amount of flame retardant should be used to maintain a certain degree of flame retardancy.